Thought I'd recommend a smart little script that is available from the Swedish company www.mallverkstan.se.
Once you have set up your variables in FM, you can update these quickly by invoking the script via File > Script > Run, which prompts you to select a tab-separated text file in two columns, where column 1 is the variable name and column 2 is the variable definition.
A typical user scenario could be to update translated manuals with translated software strings. Obviously you can import variables via File > Import > Variable Definitions, but this script lets you maintain variables in, e.g., Excel that you then save as a *.tsv file. I don't think the script is actively marketed, but should be available by contacting the company. The script is called "VariableImportFrameMaker". I've tested it in FM2020 (v126.96.36.1992) where it works fine.
Mats B (no affiliation with Mallverkstan).
IMHO the mentioned script (I can not read Swedish, ...) works in Indesign, not in FrameMaker. There is no "File > Import > Variable Definitions" menu item in FrameMaker.
As the OP points out, any script would have to offer more than FM's built-in bulk:
File » Import » Formats…
☑ Variable Definitions
Allowing the user to pick&choose individual variables would be useful, if the script runs in FM, of course.
In my work, having a template.fm for the project, containing all the key project Vars, has allowed the built-in bulk-import to suffice.
My personal view is that this script offers an advantage by acting directly on an external text file (*.tsv) and an active FM file, especially if you deal with hundreds or thousands of user interface strings that need to be switched to translated versions once a set of translated manuals are arriving from the LSP. Maintaining variables (especially user interface strings) in Excel is a strong case. I don't think the script replaces the File > Import > Formats > Variable Definition workflow for all user scenarios, but for user interface strings I think it does - and is better, as the user interface strings localization workflow is typically often Excel-centric (or in formats that are easly ported to Excel).
The FM script is not marketed on their website - and yes, they also have a similar InDesign script. You need to e-mail the company to get the FM script, as it is not actively maintained.
Regarding the other comment, I meant ofcourse File > Import > Formats > Variable Definitions - my mistake!